The present invention relates to a technique for processing a slice sample for a transmission electron microscope (TEM) using a focused ion beam device.
A sample for a transmission electron microscope is to be observed as an image of an object through which electrons have passed, which means that there is a need to process that sample to an extent that it becomes extremely thin. Production of a cross sectional sample such as a wafer by a slicing process using a focused ion beam device as shown in FIG. 6 is well known, and the following two processes are well known.
1) Mechanically cutting away a small piece form a wafer-shaped sample as shown in FIGS. 5Axe2x80x945C and processing this piece.
2) Subjecting a wafer directly to etching with a focused ion beam device, and cutting away a sliced sample.
The former process involves (FIG. 5(a)) first of all cutting a small block having a width of 500 xcexcm-2 mm and a length of 3 mm from a wafer to be sampled and then performing mechanical processing to shave off a an upper section by a further 50 xcexcm or less (FIG. 5(b)) spraying an aromatic gas or W(CO) 6 using a gas gun 14 onto central process sections of this small block to form a protective film (FIG. 5(c)). After that, a focused ion beam is irradiated to carry out slicing processing, and (FIG. 5(d)) the sliced sample is used as a cross sectional sample for TEM observation by passing an electron beam. The not yet processed sections of the small block also serve as a sample platform. The latter process is not a mechanical process, and executes focused ion beam processing from a direct wafer using a focused ion beam device. This method involves first of all forming a protective film on processing sections using a gas gun 14. A focused ion beam 12 is irradiated from above to the surface of the sample 1, both sides of the observation cross section are shaved off using a sputtering process, as will be understood from FIG. 4, and square holes 3, 4 are formed in both sides of flake sections 2 of the observation cross section. The size of the holes is such that the front hole 3 is of a size that allows the sample cross section to be observed with a scanning ion microscope by titling the sample platform, and the rear hole 4 is of a size the width is the same as that of the front hole 3 while the depth is {fraction (2/3 )}that of the front hole 3. FIG. 4 is a microscope observation subject looking from diagonally above the slice process sections 2 of a sample 1 with a scanning ion microscope.
With the method of mechanically cutting away a small piece and processing this piece as in method 1) noted above, and with the method of subjecting a wafer directly to etching and cutting away a sliced sample as in method 2), noted above, the method for final slicing processing is carried out by irradiating a focused ion beam from above towards one end of the sliced sample. This processing is finishing processing, and since it is desirable to make the damage caused by the focused ion beam extremely low, lately it is being carried out with a low acceleration voltage. However, this processing with a low acceleration voltage has a slow etching rate and thus a long processing time, and also a problem in that beam sag becomes large, positional precision of the beam is lowered causing degradation in image resolution, and processing region setting becomes difficult.
Incidentally, beam sag in the case of beam irradiation with a low acceleration voltage of 8 kV results in a positional deviation of about 0.4 xcexcm with respect to the set position. However, with a focused ion beam, constriction of ion flow into a beam shape is such that actual ion beam density has a peak in the beam center and a gaussian distribution at the periphery, as shown in FIG. 2. Accordingly, the related art is directed to processing in which a focused ion beam is irradiated from above towards an end of a slice processing section 2 and finally subjected to slicing processing, and a deviated part of this is anticipated and scanning regions in front of a processing surface are set so that sections where the ion beam density is extremely high are not irradiated to the top surface of the sliced sample. An irradiation region SA in the case shown in FIG. 1A is execution of positional setting with an estimated beam sag from the surface of the slice processing section 2 of 0.4 xcexcm. Because of this, as will be clear from the ion density distribution of FIG. 2, this processing does not use a peak value of an ion beam having good processing efficiency (high etching rate), and processing becomes executed using edge sections of the beam. This means that processing time is prolonged which is inefficient.
The present invention is directed to a method for executing slicing processing of a sample using a focused ion beam device, and has as its feature to provide a method, in an operation with a low acceleration voltage, that can carry out processing in a short time without beam sag.
The present invention adopts a TEM sample slicing process for observation of specified points on a cross section of a wafer-shaped sample, comprising a step of depositing a thick protection film on the sample surface at regions of the cross section to be observed, a step of hollowing out a large hole in front of the regions of the cross section to be observed, a step of forming hollowing out of a hole behind the regions of the cross section to be observed and forming slicing process sections, and following on from that, executing slicing processing by setting irradiation regions at regions including the slicing process section at the center and irradiating a focused ion beam from above the sample surface, using an angle of incidence/etching rate characteristic for a focused ion beam.